The present invention relates to a double-side display structure for transparent organic light emitting diodes (OLEDs) and a method of manufacturing the same.
OLEDs are generally made by vacuum sputtering an organic material and a metal film on a substrate. A conventional OLED can emit light only from a single side. It is commonly used on handsets, PDAs, digital cameras, and the like. Recently, OLED also has been used on display devices similar to projecting films that may be rolled like a curtain when not in use or become a transparent OLED to be extended on the windshield or mirror of cars for head-up display applications so that drivers can view road maps on the windshield while driving. In order to make the OLED meeting the requirement of transparent cathode structure, there is a need to develop a transparent cathode structure to serve as a double-side display OLED element.
At present the techniques of the transparent cathode structure can be grouped in two types. The first type directly plates a plurality of thin (a few nm) metal and inorganic films on an electron transport layer (such as Alq), such as U.S. Pat. No. 5,739,545, entitled xe2x80x9cOrganic light emitting diodes having transparent cathode structuresxe2x80x9d. In this patent, the transparent cathode structure consists of a low work function metal in direct contact with the electron transport layer of the OLED covered by a layer of wide bandgap semiconductor. Calcium is the preferred metal because of its relatively high optical transmissivity for a metal and because of its proven ability to form a good electron injecting contact to organic materials. However, such a transparent cathode structure with Ca (5 nm)/ZnSe (20 nm)/Al (5 nm) films plating on the Alq layer does not has an electron injecting layer to increase the efficiency. After light passes through various films (Ca/ZnSe/Al), the optical transmissivity drops and illumination of the lighting element decreases. Moreover the outmost Al film is easily affected by moisture. As a result, corrosion occurs and elements tend to degrade.
The technique of the second type transparent cathode structure is to plate an organic protection film (such as BCP, or CuPc) on the electron transport layer (such as Alq), then plate a transparent and conductive Indium Tin Oxide (ITO) film thereon. U.S. Pat. No. 6,420,031, entitled xe2x80x9cHighly transparent non-metallic cathodesxe2x80x9d discloses such a technique. The highly transparent non-metallic cathode of the OLED employs a thin CuPc film (copper phthalocyanine) capped with a film of low-power, radio-frequency sputtered indium-tin-oxide (ITO). The transparent cathode structure has two films CuPc/ITO plated on the Alq layer. Such a structure has a shortcoming, i.e. electrons have to pass over a great energy level difference (about 1.2 eV) when being transported from the cathode ITO to the organic protection film CuPc. Transportation of electrons is more difficult, as shown in FIG. 1.
Therefore the primary object of the invention is to resolve the aforesaid disadvantages. The invention employs a transparent cathode structure to replace the conventional opaque cathode structure for making a double-side transparent OLED. When the transparent OLED element emits light, the light projects from two sides of the element (anode and cathode) to form a double-side lighting display device.
In order to achieve the foregoing object, the transparent cathode structure of the invention includes an organic protection layer, an electron injecting layer, a thin metal layer, and a transparent conductive film. The organic protection layer is to protect various organic layers of the OLED elements from being damaged caused by direct electron bombardment during ITO sputtering. The electron injecting layer and the thin metal layer of the mating energy level can increase electron injecting efficiency. The transparent conductive film is to increase the conductivity of the electrode and protect other films from corrosion. According to the invention, electron transportation from the transparent cathode ITO to the electron injecting layer LiF/Al passes over an energy level difference of merely about 0.5 eV. Thus electron transportation is smoother, and element lighting efficiency increases.
The foregoing, as well as additional objects, features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: